block: Use pointer to backing_dev_info from request_queue

We will want to have struct backing_dev_info allocated separately from
struct request_queue. As the first step add pointer to backing_dev_info
to request_queue and convert all users touching it. No functional
changes in this patch.

Change-Id: Ib99961747ed2d58cc2f9c0581c66c55a49aaee4b
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Git-commit: dc3b17cc8bf21307c7e076e7c778d5db756f7871
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


[riteshh@codeaurora.org: Resolved minor merge conflicts]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>